Data Sheet

UB20.241
UB-Series
24V, 20A, DC-UPS
23.2. EXTERNAL INPUT PROTECTION
The DC-UPS is tested and approved for branch circuits up to 28A. An external protection is only required, if the
supplying branch has an ampacity greater than this. If an external fuse is necessary or utilized, minimum requirements
need to be considered to avoid nuisance tripping of the circuit breaker. A minimum value of 25A B- or C-Characteristic
breaker should be used.
Check also local codes and local requirements. In some countries local regulations might apply.
23.3. OUTPUT CIRCUIT BREAKERS
Some applications require branch circuit or branch circuit conductor protection on the 24V supply voltage. Therefore
standard miniature circuit breakers (MCB’s or UL 1077 circuit breakers) are commonly used on 24V branches.
MCB’s are designed to protect wires and circuits. If the ampere value and the characteristics of the MCB are adapted to
the wire size that is used, the wiring is considered as thermally safe regardless of whether the MCB opens or not.
To avoid voltage dips and under-voltage situations in adjacent 24V branches which are supplied by the same source, a
fast (magnetic) tripping of the MCB is desired. A quick shutdown within 10ms is necessary corresponding roughly to
the ride-through time of PLC's. This requires high peak currents to open the circuit breaker in the required time.
Furthermore, the impedance of the faulty branch must be sufficiently small in order for the current to actually flow.
For this discipline, two cases must be differentiated:
A) DC-UPS is operating in power supply mode:
In this case, the supplying source is responsible for the peak current to trip circuit breakers. These values can be
found in the datasheet of the power supply.
B) DC-UPS is operating in battery mode:
The following table has typical test results showing which B- and C-Characteristic MCBs magnetically trip
depending on the wire cross section and wire length (setting for buffer voltage: 22.5V).
Fig. 23-3 Test circuit
Maximal wire length
*)
for a fast (magnetic) tripping:
0.75mm² 1.0mm² 1.5mm²
2.5mm²
C-2A
25m 34m 48m 72m
C-3A
24m 32m 47m 68m
C-4A
18m 24m 33m 48m
C-6A
10m 12m 18m 28m
C-10A
5m 6m 9m 13m
C-13A
1m 2m 2m 3m
B-6A
23m 26m 32m 46m
B-10A
10m 14m 20m 31m
B-13A
9m 10m 18m 27m
MCB
DC-UPS
Output
+
-
+
-
Load
Wire length
S1... Fault simulation switch
S1
B-16A
3m 4m 9m 13m
*) Don’t forget to consider twice the distance to the load (or cable length) when calculating the total wire length (+ and – wire).
23.4. BACK-FEEDING LOADS
Loads such as decelerating motors and inductors can feed voltage back to the DC-UPS. This feature is also called return
voltage immunity or resistance against Back- E.M.F. (E
lectro Magnetic Force).
This DC-UPS is resistant and does not show malfunctioning when a load feeds back voltage to the power supply. It
does not matter whether the DC-UPS is on or off.
The maximum allowed fed-back-voltage is 35Vdc.
Feb. 2016 / Rev. 2.1 DS-UB20.241-EN
All parameters are specified at 24V, 20A output current, 25°C ambient and after a 5 minutes run-in time unless otherwise noted.
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